1. Field of the Invention
The present invention relates to an automatic stop device for use in a screw striking machine and, in particular, to an automatic stop device for use in a screw striking machine which automatically stops an air motor when a screwing operation is completed.
In addition, the present invention relates to an automatic stop device which can automatically stop the operation of the air motor when the screwing depth of the screw reaches a given depth especially in a condition that a screw is struck onto a member to be screwed in a floating condition by a screw striking machine and the screw is then rotated and tightened.
2. Description of the Related Prior Art
Conventionally, a screw striking machine uses high pressurized air as a power source. A driver bit connected to a piston of an air cylinder is driven and rotated by an air motor to thereby strike a screw into a member to be screwed. In this type of screw striking machine, as a safety device, a contact arm is disposed and projected from the nose portion of the screw striking machine. The contact arm is energized by a spring in a direction where the screw is struck. Due to the contact arm, a trigger lever is operable when the leading end portion of the contact arm is pressed against the surface of the member to be screwed and the other end portion of the contact arm is butted against a contact arm stopper disposed within the screw striking machine.
If the trigger lever is actuated, then a piston within the air cylinder is rapidly lowered; and, in response to this, the driver bit connected to the piston drives a screw into the member to be screwed such as a building material or the like to a certain degree and, at the same time, the locking of the contact arm by the contact arm stopper is removed, so that the air motor drives and rotates the driver bit to thereby screw the screw into the member to be screwed. If the operation of the trigger lever is removed, then the operation of the air motor is caused to stop, and thus the piston and driver bit respectively move upward to return back to their wait positions.
As for another kind of a screw striking machine, when a screwing operation is completed, the operation of an air motor is automatically stopped to thereby control the screwing depth of a screw to a constant depth. In an automatic stop device for use in the screw striking machine of this type, the device includes an open/close valve in an air supply pipe passage to an air motor, and switches the open/close valve by a contact arm to thereby stop the operation of the air motor automatically.
In particular, if the screw is rotationally driven by the air motor and is thereby screwed into the member to be screwed, then a distance between the screw striking machine and the member to be screwed decreases, and the contact arm is further pushed in toward the main body side of the screw striking machine. In the above-mentioned automatic stop device, at the time of completion of the screwing operation, the contact arm presses against the stem of the open/close valve to thereby switch the open/close valve over to its pressurized air cut-off position, thereby stopping the operation of the air motor.
In another type of automatic stop device, a device is also structured such that, if a piston connected to a driver bit reaches its bottom dead point or one end of its travel as a screw is screwed, then a seal member attached to the piston cuts off an air supply passage to an air motor to thereby stop the operation of the air motor.
When striking a screw into a member to be screwed with the screw striking machine, in order for the driver bit to be able to drive and rotate the screw positively, it is necessary to press the contact arm of the screw striking machine against the member to be screwed with a certain degree of pressure. However, if the pressing load is excessive, then the driver bit and piston are pushed in toward the main body side of the screw striking machine prior to completion of the screwing operation, with the result that the contact arm is also pushed in. In particular, in a screw striking machine of a type that an open/close valve can be switched by the contact arm, if the above-mentioned state occurs, then there arises a problem that the operation of the air motor is caused to stop prior to completion of the screwing operation.
On the other hand, if the load with which the screw striking machine is pressed against the member to be screwed is too small, then there is a possibility that the screw striking machine can be removed from the surf ace of the member to be screwed due to a reactive force produced when the screw is struck by the screw striking machine. In a screw striking machine of a type that, when a piston reaches its bottom dead point, an air supply passage to an air motor is cut off, if the above-mentioned state occurs, then the piston reaches the bottom dead point prior to completion of the screwing operation, thereby causing the operation of the air motor to stop.